The present invention relates generally to drawing metal tubes and particularly to apparatus and a method for providing precisely located thick and thin wall portions in elongated drawn tubes in an economical manner.
AA recent development employing step wall tubing is disclosed in a U.S. Pat. No. 4,527,978 to Zackrisson. In this patent, a yoke for a universal joint is welded in each end of a hollow tube, each end having a wall that is somewhat thicker than the wall intermediate the ends. The tube with the welded yokes provides a drive shaft for use in motor vehicles.
Automotive drive shafts are one of the many uses for long lengths of drawn tubing that have been provided with thick and thin wall portions. For example, such spaced apart thick wall portions along a tube length can provide structural areas for many types of fastening means when members need to be fastened to the tube. Further, such a tube can weigh less than a straight wall tube. Or, if a particular portion of the tube requires that its outer diameter be turned down (i.e. machined), the thickness of the tube wall can be controlled and maintained.
If thick wall portions are located internally of a long tube such that the thick portions are not externally visible to the untrained eye, the user of such tube lengths must be assured of the locations of the thick wall portions. If an indication of the location of the internal thickwall portions appears on the external surface of the tube, then personnel knowing what to look for will be able to locate the position of the thickwall wall portion. In the case of making short lengths of tube from long tube lengths by cutting the same into short lengths, such as the drive shafts of Zackrisson, the locations of the thick wall portions must be precisely known in order to position the cutting mechanism at the precise longitudinal center of each thick wall portion so that each end of the resulting tube section will have the same length of thick wall.
It is, therefore, an objective of the invention to precisely locate thick and thin wall portions along the lengths of drawn tubes.
A problem that has been encountered in making step wall tubing is the chattering of the drawing bulb in the mouth of the die when the bulb is inserted to provide the thin wall portions. Chattering is caused by a force component in the die mouth that resists seating the bulb while friction between the tube and bulb, as the tube moves through the die, attempts to seat the bulb in the die mouth. If the bulb does not immediately and properly seat in the mouth of the die and stay seated during the drawing process, the chattering bulb marks the internal surface of the tube with a series of rings and indentations. Such rings and indentations can function as stress risers to cause future weakness problems when the tube is used.
What is therefore needed in the industry and which forms another primary objective of the present invention is the provision of drawing apparatus that prevents bulb chattering and provides a smooth transition area between the thick and thin portions of the step wall tube.
Previous practice by the present inventors to make step wall drawn tube involved manually manipulating a standard drawbench by starting, stopping and reversing drawbench action. This resulted in a severe increase in cycle time over that for drawing a similar length of straight wall tube. For example, the cycle time for a single step tube was 200% greater than that of a straight wall tube, and 1500% greater for a 12 step tube. Further, accuracy at best was plus or minus 5 inches which resulted in 100% to 1000% increase in sawing time, as the locations of the thick and thin wall portions were uncertain, i.e., it was difficult to properly locate the tube relative to the saw. This resulted in 25% of the stepped sections to be unusable. In addition, such low accuracy prevented the repeatability from tube to tube needed for applications of multiple stepped products. Steps closer together than 18 inches could not be made with the manual method, which resulted in scrapping excess tube length and prevented fulfilling the market demand for short step tube and some multi-step tube.
It is therefore another objective of the invention to make step wall tube as economical or more economical than straight wall tubing, and to provide steps short enough to meet current market demands.